1. Field of the Invention
The present invention relates to filter circuits, and more specifically to a method and apparatus for implementing filter circuits based on trans-conductor circuits.
2. Related Art
Filter circuits are generally implemented to perform corresponding transfer functions as is well known in the relevant arts. By the appropriate choice of transfer functions, filter circuits may be implemented to provide corresponding utilities (e.g, low pass filter, band pass filter). Filter circuits may be used in several environments such as signal receiving systems, digital circuits, etc., as is well known in the relevant arts.
Filter circuits are often implemented using components such as trans-conductors and operational amplifiers. In some environments, it may be desirable to implement filter circuits using trans-conductors at least in that trans-conductors generally consume less electrical power than operational amplifiers. For further information on trans-conductors, the reader is referred to a book entitled, AAnalogue IC Design: The current-mode approach@, by C. Toumazou, F. J. Lidge and D. G. Haigh, ISBN No.:086341 215 7, and is incorporated in its entirety herewith.
However, one typical problem with trans-conductor circuits based filter circuits is that the noise components introduced into the output signals is generally high at least compared to operational amplifiers based circuits. One way to reduce the noise component is by operating the filter circuits at high voltage levels or high power levels. However, such operation generally consumes more electrical power, and may be undesirable at least in some environments. In addition or in the alternative, the area (on an integrated circuit) of the trans-conductor circuits may have to be increased to reduce the noise factor, which may also be undesirable in many environments.
What is therefore required is a filter circuit based on trans-conductor circuits which satisfies one or more of the above-noted requirements.
A basic block implemented according to an aspect of the present invention may be used to implement a filter circuit. In an embodiment, the basic block contains a biasing circuit providing a biasing signal to set an operating point of a trans-conductor circuit and a common-mode feedback circuit providing a feedback signal to stabilize the trans-conductor circuit, with the biasing signal and the feedback signal being combined and provided on a common path to the trans-conductor circuit.
Due to the use of the common path, the number of components (including transistors) to implement the basic block may be reduced. As a result, the noise introduced by the components of a filter circuit and the total electrical power consumed may be minimized.
An embodiment of the basic block may further include a control circuit generating a quiescent voltage of the trans-conductor circuit, with the control circuit being implemented to have a same (or similar) transfer function as the trans-conductor circuit. The biasing circuit may set the operating point of the trans-conductor circuit based on the quiescent voltage also. In one implementation, the control circuit is implemented similar (in terms of components and connectivity) to the trans-conductor circuit and operated on by using a D.C. voltage to generate the quiescent voltage of the trans-conductor circuit.
The common-mode feedback circuit may contain a common mode sense circuit generating a common mode voltage, and an error amplifier may amplify the common mode voltage and providing a resulting amplified output to the biasing circuit. In an embodiment, the trans-conductor circuit is implemented using PMOS transistors and the biasing circuit is implemented using NMOS transistors.
Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.